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AIR-LIFT PUMPS Principle of Working of Air-lift Pumps

An air-lift pump operates by the injection of compressed air directly into the water inside a discharge or eductor pipe at a point below the water level in the well. The operating principle of the pump is shown in Fig. 8. The injection of the air results in a mixture of air bubbles and water. This composite fluid is lighter in weight than water so that the heavier column of water around the pipe displaces the lighter mixture forcing it upward and out of the discharge pipe. The piping assembly used for air-lift pumping from a well consists of a vertical discharge pipe called educator pipe and a smaller air pipe.

An air-lift pump for pumping or developing a well is arranged commonly with the air pipe inside the eductor pipe as shown in Fig. 8. It is also possible to locate the air pipe outside the eductor pipe if there is space in the bore hole, as otherwise there is considerable friction loss when the air pipe is located inside a small diameter eductor pipe. Both the eductor pipe and the air pipe must be submerged in water in the well with 40% or more of their lengths extending below the pumping level. The energy that is available to operate the air-lift pump is that which is contained in the compressed air.

The well casing itself can be used for the eductor pipe if the diameter of the casing is not much larger than the air line. This provides a practical way to pump sand and mud from the bottom of a well during development and cleaning operations. When test-pumping a well, however, it is better to use a separate educator pipe since the pumping level can then be measured with steel or electronic tapes.

Principle of operation of an air-lift pump

Fig. 8. Principle of operation of an air-lift pump.

The two most important factors in air-lift pumping are the per cent submergence of the airline and the relative sizes of the air and eductor pipes. Generally, the air-lift pump works best with a submergence of 60.6 or more. It is the length of airline that is submerged below the pumping level divided by the distance from the level of water discharge to the lower end of the airline and multiplied by 100, to give the result as percentage. Referring to Fig. 8

The length of air pipe below the static water level is significant only for determining the air pressure required to start the air-lift pump. Before air can discharge from the lower end of the air pipe, the compressed air must push all the water out of the air pipe. To do this, the air pressure must be greater than the water pressure before starting to pump. The depth of water in metres from the static water level to the lower end of the air pipe divided by 10 gives the required air pressure in kg/cm2.

A + D

Thus, starting pressure (kg/cm2) =

A

Working presume (kg/cm2) = = in which A and D are measured in metres.

Pipe sizes for air-lift pumps: The velocity of water in the eductor pipe depends upon the volume of air and water being discharged and the area of the annular space between the airline and the eductor. Table-1 gives the size of airlines for various sizes of eductor used under most conditions. When the water yield is unusually low, the difference between the sizes of eductor and air pipes may have to be less to reduce the area of the annular space and thus increase the discharge velocity enough to prevent excessive air slippage in the eductor. This condition necessitates a larger-than-normal air line when pumping directly from the well casing without a separate eductor pipe.

Table 1. Pipe Sizes for Air-lift Pumps

Pumping rate (litres 1 sec)

Size of well casing eductor pipe (cm)

Size

(cm)

Internal diameter

Size

of air line (cm)

2to4

10.0 or large

5.0

1.25

4to5

12.5 or larges,

7.5

2.50

5 to 6

15.0 or largely

8.7

2.50

6to9

15.0 or larger

10.0

3.13

9 to 16

20.0 or large

12.5

3.75

16 to 25

20-0 or larger

15.0

5.00

25 to 44

25.0 or larger

20.0

6.25

Best operation of an air-lift pump requires good regulation of the amount of air injected. Too much of air causes excessive friction in the pipe lines and waste of air. Too little air results in reduced yield and in surging, intermittent discharge. When developing a well with an air-lift pump, the discharge should be started at a very low rate and brought up gradually. The air flow should be slowly increased in proportion to about what appears to be the increase in water flow into the well from the water-bearing formation.

Air-lift pumping is extensively used in the development and preliminary testing and cleaning of tube wells. It is also sometimes used in crooked tube wells where vertical turbine pumps or submersible pumps cannot be installed. The advantages of air-lift pumps are: simplicity, tube well need not be perfectly straight or vertical, and impure water will not

damage the pump. The main disadvantage is its low efficiency. The maximum efficiency usually obtained is about 30 per cent. The initial cost of the pump, including the air compressor, is high and it requires an extra depth of water for proper submergence.

SELECTION OF PUMPS

Irrigation wells and pumps are costly installations which require efficient utilization. A major part of the energy used in agriculture is in pumping. The total energy utilized in irrigation pumping in India in the year 1973-74 was about 8,400 million kilowatt-hours. It was estimated that the requirement would double in the next two decades. Efficient utilization of limited energy resource calls for the selection of the most suitable pump, keeping in view the requirement of irrigation, characteristics of the well or other sources of water, kind of power available, economic conditions of the farmer and other factors.

Criteria for selection of irrigation pumps: The main factors influencing the selection of pumping sets are:

  • (i) The retirement of irrigation water by the crops to be irrigated,
  • (ii) Yield of the sources of water (open wells, tube wells, streams rivers, ponds) and
  • (iii) Availability and cost of the pump and kind of energy.
 
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